* The H-R diagram plots luminosity and surface temperature: These properties are derived from the light emitted by stars. Black holes and neutron stars don't emit their own light. They primarily interact with their surroundings through gravity, causing the accretion of material, which then emits light (think of the accretion disks around black holes). This light is not representative of the object's own properties, but rather a by-product of the accretion process.
* H-R diagrams focus on stars in active stages: The H-R diagram mainly represents stars that are fusing hydrogen or heavier elements in their cores. Black holes and neutron stars represent the remnants of collapsed stars after they have exhausted their nuclear fuel. They are the end products of stellar evolution, not active stars themselves.
So, while black holes and neutron stars are fascinating and important objects in the universe, they don't fit neatly into the framework of the H-R diagram.
However, astronomers do have ways of studying them and representing their characteristics:
* Mass-radius diagrams: These diagrams are more relevant for compact objects. They plot the mass of a celestial object against its radius. Neutron stars and black holes would occupy distinct regions on such a diagram.
* Evolutionary tracks: While black holes and neutron stars don't appear directly on the H-R diagram, their formation can be depicted by extending evolutionary tracks from massive stars towards the endpoint of stellar evolution.
It's important to remember that the H-R diagram is a powerful tool, but it focuses on a specific aspect of stellar evolution. Black holes and neutron stars are fascinating objects that require different tools and approaches to study and understand.
